1. Field of the Invention
The present invention relates to a silicon film and a method for making the same, and more particularly, to a semiconductor silicon film formed by deposition and utilizing solid silicon as a raw material, and its fabrication method.
2. Discussion of the Related Art
A silicon film has been used in the fabrication of thin film solar cells and the like. A conventional silicon thin film solar cell comprises a hydrogenated amorphous silicon film (a-Si:H) fabricated by a chemical vapor deposition (CVD) process onto a glass substrate. A problem with such amorphous silicon solar cells is a gradual decrease in efficiency upon prolonged exposure to light. An attempt to solve this problem involves a high temperature heat treatment to crystallize the a-Si:H films. An additional drawback of the CVD process is its utilization of silane gas which is volatile and increases the risk of fire.
Electronic beam deposition is an easier method of fabricating a silicon film, wherein an electronic beam gun evaporates solid silicon and the silicon vapor is deposited onto a substrate. However, silicon films obtained by this method contain many dangling bonds that deteriorate its semiconductor properties and are inferior to the silicon films obtained by the CVD process. Hydrogen atoms have been effective to terminate the dangling bonds and decrease their occurrence.
FIG. 8 illustrates schematically a conventional apparatus used in an electronic beam deposition method. The apparatus includes an exhaustible vacuum chamber 51, an evaporation source 53 including an electronic beam gun, a heater 60, a substrate 61 being heated by the heater 60, an ion gun 55 for supplying hydrogen ions in the chamber 51, and an exhaust pipe 57. The air in the chamber is pumped out through the exhaust pipe 57 such that the chamber 51 exhibits a vacuum to a predetermined degree. The electronic beam, generated by the electronic beam gun and directed to the evaporation source 53, evaporates a raw silicon material, while hydrogen ions are generated by hydrogen gas produced by the ion gun 55. The silicon vapor, produced by the evaporation source 53, and the hydrogen gas, from the ion gun 55, reach the substrate 61 at substantially the same time and combine to form an a-Si:H film. Hydrogen ions effectively decrease the density of the dangling bonds in the a-Si:H films from a density on the order of 10.sup.20 cm.sup.-3 without any hydrogen ions to 3.times.10.sup.17 cm.sup.-3 with hydrogen ions.
While the deposition method improves the properties of the silicon films by utilizing hydrogen ions, it limits the speed of forming an a-Si:H film because of the insufficient current density of the hydrogen ions. In a conventional ion gun, hydrogen ionizes and an electrical grid conducts the generated ions to produce and conduct current through the chamber 51. A space charge limits the ion current and makes it difficult to produce a large ion current.